Process for the production of a fibrous polyamide laminar structure



3,271,237 PROCESS FOR THE PRODUCTION OF A FIBROUS POLYAMIDE LAMINAR STRUCTURE Erwin Summer and Klaus Gerlach, Obernau am Main,

and Klaus Boehme, Erlenbach am Main, Germany, assignors to Vereinigte Glanzstofi-Fabriken AG., Wuppertal-Elberfeld, Germany No Drawing. Filed Sept. 15, 1964, Ser. No. 396,729 Claims priority, application Germany, Sept. 17, 1963, V 24,598 9 Claims. (Cl. 162-146) The invention relates to a process for the production of a fibrous polyamide laminar structure such as paper or non-woven textile fabrics, web-s, fleeces or sheets having a relatively smooth surface. The invention also relates to the products obtained by the process, including both paper and textile types of products. More particularly, the invention is concerned with the production of such non-woven fibrous products on a papermaking machine, wherein the fibers consist essentially of a polyamide.

It is well known that synthetic fibers can be waterlaid form an aqueous dispersion onto a papermaking machine in order to form paperlike laminar structures, and polyamide fibers have been added to cellulose fibers in order to improve the strength of a cellulosic paper. In some cases, an attempt has been made to produce paperlike products consisting only of synthetic fibers, but there are many difiiculties in processing such synthetic fibers on a conventional papermaking machine without using various auxiliary substances capable of acting as binding agents and/or severely modifying the structural elements of the paper-making machine. These problems arise for the most part because a waterlaid web of normal melt spun and non-fibrillated staple fibers cannot be dried with substantial removal of water in the drying zone of the conventional papermaking process Without disintegrating or falling .apart. By comparison to cellulose fibers, synthetic polyamide fibers do not have any binding capacity and are therefore incapable of forming a coherent waterlaid web which will stick together during drying.

A number of techniques have been employed in order to obtain a satisfactory paperlike product composed of synthetic fibers by using special binding agents or specially treated fibers. For example, water-soluble substances having adhesive properties or dispersible synthetic resins have been admixed with the aqueous dispersion of fibers in order to provide a binding agent capable of coating the fibers and adhering them to each other. Such binding agents are unsatisfactory because there is a very low retention of the binder on the fibers and there must be a correspondingly high concentration of the binder in the aqueous dispersion, often considerably higher than the fiber concentration. Under these cirumstanms, there is usually a severe loss of the binding agent during the waterlaying step. In addition, in coating the fibers with a sufiicient amount of the binding agent to cause good adhesion and a strong web, one cannot obtain a textile type product having a soft or attractive handle, especially where the binding agent is a chemical substance quite alien to the synthetic fibers. The physical properties of the paperl-ike product are thus determined to a large degree by the properties of the binding agents, thus detracting from the otherwise desirable properties of the synthetic fibers.

Other processes have been described in which the fibrous web contains a binding agent cap-able of melting and adhering the fibers at temperatures substantially above that which can be achieved in the drying zone of conventional paperrnaking machines. These binding agents, including meltable fibers, are incorporated in the States Patent initial waterlaid web which is then dried and then transported to a heated calender where the binding fibers are melted in a second operation. It will be apparent that this technique requires a second melting step to be combined with the conventional papermaking process and requires the use of other binding agents as discussed above, since the waterlaid web must first be conducted through the papermaking process under conditions which are not sufiicient to melt the binding fibers and permit them to form a coherent web.

The difiiculties in using such binding agents have been avoided in another known process in which so-called fibrids are waterlaid together with other synthetic fibers. (See United States Patent No. 2,999,788.) These fibrids are synthetic polymers having an irregular fibrous structure such as are obtained by precipitation of dissolved polymers with suitable precipitating agents. The fibrids tend to hook together during waterlaying of the fibrous web or fleece, and the resulting web can be dried in the normal drying zone of the papermaking process without losing its cohesion. If desired, the fibrids can then be melted out of the Web in a second operation while form ing a solid paperl-ike product. Although this use of fibrids permits a relatively easy processing of synthetic fibers into paperlike products on a papermaking machine, the production of the fibrids themselves is a very elaborate and expensive procedure.

One object of the invention is to provide an inexpensive and simple method of producing a laminar product consisting essentially of non-woven synthetic polyamide fibers.

Another object of the invention is to provide an efiective process for producing fibrous polyamide laminar structures in accordance with the conventional papermaking process of waterlaying and drying a fibrous web. Still another object of the invention is to provide a process for the production of fibrous polyamide laminar structures in which the usual binding agents can be avoided while still obtaining a strong, cohesive, non-woven fibrous polyamide web.

Yet another object of the invention is to provide a process for the production of fibrous polyamide laminar structures in which essentially only polyamide fibers can be waterlaid from an aqueous dispersion and dried by the conventional papermaking process at relatively low temperatures without tearing or disintegrating the fibrous product.

Another object of the invention is to provide a process in which polyamide fibers can be waterlaid and dried to form a strong-coherent fleece or web capable of being formed into a. variety of laminar structures with different surface characteristics as might be found in pape-rlike, leatherlike or feltlike products.

These and other objects and advantages of the invention will become more apparent upon consideration of the following deatiled description of the invention.

It has now been found, in .accordance with the invention, that a fibrous polyamide laminar structure can be advantageously produced on a papermaking machine by waterlaying polyamide fibers from an aqueous dispersion onto the foraminous support of the papermaking machine, provided that the fibers consist essentially of 10 to 100% by weight of a copolyamide of caprolactamzhexamethylene adipamide in a weight ratio of about 2:3 to 3:1, and to 0% by weight of a polyamide selected from the group consisting of polycaprolactam and polyhexamethylene adipamide. Water is removed from the fibers on the foraminous support to form a non-woven waterlaid web, and the waterlaid web must then be heated in a moist state to a temperature sufiicient to soften the copolyamide fibers and cause them to become adherent, thereby providing a cohesive or interlinked fibrous laminar structure. It is thus possible to produce a laminar structure on a papermaking machine wh ch contains only polyamide fibers with melting points which .lie above 165 C. if an aqueous dispersion of fibers is employed wherein the total content of fibers consists of to 100% by weight of the copolyamide fiber which is then capable of coalescing or adhering the waterlaid web or fleece into a cohesive laminar structure when subjecting the waterlaid web in a wet or moist state to a heat treatment at relatively low temperatures, eg about 80 to 100 C. The dispersion of polyamide fiber in the form of an aqueous slurry or pulp can be processed in conventional manner on a papermaking machine into a waterlaid web and subsequently dried for removal of adherent water, but it is essential during this conventional procedure to heat the moist copolyamide fibers to a temperature at which they tend to soften and stick to each other or to the other polyamide fibers.

The term polyamide is employed herein solely with reference to the fiber-forming polyamides, and more particularly to those polyamides obtained by polycondensing caprolact-am, hexamethylene adipamide and mixtures thereof. Polyhexamethylene adipamide, sometimes referred to as nylon 66, is most easily obtained by polycondensing hexamethylene adipamide under well known conditions leading to a fiber-forming polymer. Hexamethylene adipamide may also be briefly described as the AH salt since this salt of hexamethylene diamine and adipic acid is normally employed as the monomeric reactant. This AH salt has also been named hexamethylene diammonium adipate. Polycaprolactam, sometimes referred to as nylon 6, is of course obtained by polycondensation of caprolactam, and the production of polycaprolactam fibers is also a well known procedure in the art. Poly-hexamethylene adipamide and polycaprolactam fibers are completely interchangeable or can even be used as mixtures for the purposes of the present invention.

It is possible to polycondense caprolactam and hexamethylene adipamide in any molar or Weight ratio and to spin the resulting copolymer or copolyamide into fibers with melting points which lie between about 168 C. and 256 C. However, these fibers have a special property in that they begin to soften at considerably lower temperatures in the presence of water or saturated steam, i.e. in the moist state. Depending upon the composition of the copolyamide, it is even capable of softening and becoming adherent in boiling water. In this respect, the term softening refers to the temperature range which extends from the glass transition point or second order transition temperature up to the crystalline melting point or first order transition temperature of the polymer fiber. The softening point or beginning of the softening range can also be defined as that temperature at which the filament, conditioned by a sudden drop in the inner viscosity, begins to lose its original crystalline form under the influence of surface tension. For purposes of the present invention, it is important to employ a copolyamide which exhibits this property of softening in the moist state at relatively low temperatures.

The copolyamide fibers (A) are preferably obtained by melt spinning a copolycondensate of about 40 to 75% by weight of caprolactam and 60 to by weight of hexarnethylene adipamide (AH salt). The copolyamide fibers obtained from about 50 to 70% by weight of caprolactam and 50 to by weight of the AH salt are especially preferred. In general, the copolyamide fibers should have a melting point of about 168 to 190 C. and should exhibit a softening point in the moist state of about 75 to 100 C. Although the composition of the copolyamide fibers is goverened to some extent by the nature of the desired final product, it is essential for the purposes of the invention that these copolyamide fibers tend to adhere when heated in the moist state, and it is particularly desirable to obtain such adherence with a heat treatment at temperatures below about 110 C.

In carrying out the process of the invention, the copolyamide fibers are dispersed in water either alone or together with fibers of polycaprolactam or polyhexamethylene adipamide, and it is desirable to obtain a uniform dispersion of the fibers by adding any suitable wetting agent or surface active agent, commonly employed for this purpose. The preparation of the fiber dispersion can be accomplished in any suitable mixing tank or the head box of the papermaking machine, and conventional procedures are followed as to selecting a suitable staple length of the fibers, the proportion of fibers in the aqueous composition, the amount of wetting agent and the degree of mixing required to provide a Water-layable fibrous slurry. The fibrous dispersion or slurry is then waterlaid onto the Fourdriner or similar foraminous screen of the papermaking machine, and the waterlaid web is worked into a fibrous laminar structure by removing water and subsequently drying the moist web. All of these steps follow the conventional papermaking procedure except for certain modifications in accordance with the present invention, the most important of which involves a heat treatment of the waterlaid web in a moist state at a temperature which is sufficient to cause the copolyamide fibers to soften and become adherent. This heat treatment is preferably carried out after the moist waterlaid web has passed through the last wet press of the papermaking machine, and the moist web then acquires sufficient strength to be conducted through the drying zone of the papermaking machine. In general, during this heat treatment of the waterlaid web, it should have a moisture content with reference to the wet web of at least about 40% up to approximately by weight.

The particular technique of applying heat to the moist waterlaid web according to the heat treatment step of the invention depends in part upon the composition of the copolyamide fibers and in part upon the desired character of the finished product. For example, when using copolyamide fibers of 60% by weight caprolactam and 40% by Weight AH salt, the heat treatment can be carried out in such a manner that the moist web is conducted under a continuous felt blanket over a heated rotating roller, according to the usual operation of initially drying paper during the initial drying stage of the papermaking machine, the surface temperature of this heated roller being somewhat above C. Since these particular copolyamide fibers will soften in boiling water, the moist waterlaid web strongly coalesces or solidifies after being heated with the roller and can then be dried without ditficulty in the remaining drying zone of the papermaking machine. Also, in this manner of operation, contact pressure can be applied to the moist waterlaid web while softening and adhering the copolyamide fibers by compressing the web between the felt blanket and the heated roller. Because of this contact pressure, a paperlike laminar structure is obtained with a high tearing strength, and the paper product does not require any additional treatment.

In another heat treatment procedure according to the invention, the waterlaid moist web is conducted without any tension or contact pressure over a heated rotating cylinder maintained at a temperature sufficient to soften the copolyamide fibers. During this operation, there occurs not only a coalescence or adhesion of the fibrous web but also a substantial shrinkage of surface area, and after subsequently drying the heat treated web, a soft laminar structure is obtained with a textile or feltlike handle. Such results are especially apparent when copolyamide fibers are employed which have only a slight tendency to soften and coalesce in boiling water, for example polycondensates of approximately 70% by weight caprolactam and 30% by weight AH salt. Fleecelike products are thus obtained with such copolyamide fibers, for example in admixture with polycaprolactam fibers which preferably make up about 50 to 70% by weight of the total fiber content. The strength of this fieecelike or feltlike laminar product can be further improved by again softening or melting the copolyamide fibers in a second heating step.

It will be obvious that the heat treatment of the moist waterlaid web can be carried out in any suitable manner including the heated roller described above, by di recting saturated steam onto the waterlaid web or by using infrared radiators. it is especially advantageous to use a heated roller or cylinder because the waterlaid web in a moist state can be heat treated with or without tension and/or contact pressure in order to provide a variety of laminar products ranging in texture or handle from paperlike to fabric materials. It is also possible to subject the moist web to more than one heat treatment at substantially any point in the paper-making process after excess Water has been removed on the foraminous supporting structure and before the complete drying of the web. Various modifications of the heat treatment step will be readily apparent to those skilled in the art and such modifications are included within the spirit and scope of the invention. By using copolyamide fibers having lower softening points, one obtains laminar products which are stronger and more paperlike in their structure. Similarly, by using copolyamide fibers with higher softening points, the laminar product tends to lose its strength and to acquire softer textile properties. The appearance and structure of the laminar products can also be changed by varying the content of the polycaprolactam or polyhexamethylene adipamide fibers and also by varying the thickness of the web during its processing on the papenmaking machine. The process of the invention is thus quite fiexible in permitting the produc tion of fibrous laminar structures with different physical characteristics, surface textures, strength and porosity.

The process of the invention is further illustrated but not limited by the following examples.

Example 1 A fiber dispersion was prepared from 70 parts by weight of polycaprolactam fibers 1.4 denier, 6 mm. staple length), 30 parts by weight of a coplyamide fiber (1.4 denier, 6 mm. staple length) and 100,000 parts by weight of water with the addition of a polyoxyethyl ether of a fatty alcohol* as a wetting or dispersing agent. This aqueous fiber dispersion was then processed on a conventional papermaking machine equipped with the usual Fourdrinier screen and pressing and drying rollers combined with a continuous felt blanket. The copolyamide fiber was melt spun from a polycondensate of 60% by weight caprolactam and 40% by weight AH salt, had a melting point of 169 C. and a softening point in water of 75 C. After wet pressing the waterlaid web, the moist web was conducted under the contact felt blanket over a rotating roller, the temperature of this roller being adjusted such that the moist web was heated to about 100 C. After leaving the heated roller, the web was dried without diificulty in the drying zone of the papermaking machine.

There was obtained a porous paper of high tearing strength, having a tearing length of 8500 m. as determined according to DIN 56112 (German Industrial Standards, a standard testing method). No further after-treatment of this paper was required.

A sample of a paper prepared in the same manner with a tearing length of 7800 m. was immersed for minutes in boiling water. After drying the sample, its tearing length had not been changed.

Example 2 A fiber dispersion was prepared from 70 parts by weight polycaprolactam fibers (5 denier, 6 mm. staple length), 30 parts by weight of the copolyamide fiber of Example 1 and 100,000 parts by Weight of water. The

*Polyoxyethyholeicether.

fibers were waterlaid and processed on the papermaking machine, and after wet pressing, the still moist web subjected to the heat treatment on a roller heated to 100 C., all as described in Example 1. There was thus obtained at strong, springy, porous laminar structure with a tearing length of 6,500 m. which is particularly suitable for use as an inlay material.

Example 3 A fiber dispersion was prepared from 30 parts by weight polycaprolactam fibers (1.4 denier, 6 mm. staple length), 70 parts by weight of a coplyamide fiber of the same composition as in Example 1 1.4 denier, 6 mm. staple length) and 100,000 parts by weight of water. This fiber dispersion was processed on the papermaking machine with the same heat treatment as described in Example 1. There was obtained a transparent, porous, reinforced fibrous foil which exhibited a high tearing strength.

Example 4 A fiber dispersion was prepared from 60 parts by weight polycaprolactam fiber (1.4 denier, 6 staple length), 40 parts by weight of a copolyamide fiber melt spun from a copolycondensate of 30% by weight Ad-I salt and 70% by weight caprolactam (melting point 172 C., 2.5 denier, 6 mm. staple length) and 100,000 parts byweight of water. The fiber dispersion was processed in conventional manner on the papermaking machine, except that the moist waterlaid web or fieece, after the last wet press and before entry into the drying zone, was conducted without tension and without contact pressure over a heated rotating cylinder whose temperature was adjusted such that the moist web was heated to about 100 C. During this heating step on the cylinder, there occurred a surface area shrinkage of the web or fieece of about 20%. After leaving the roller, the web and sufficient strength to be dried in the usual manner without any substantial tension or compression. There was obtained a soft feltlike laminar product, and the strength of this product could be further reinforced by heating it in the absence of tension to a temperature above the rnelting point of the copolyamide fiber. The product was suitable for the production of a fibrous patent leather or similar materials.

The invention is claimed as follows:

1. A process for the production of a fibrous polyamide laminar structure on a papermaking machine which comprises: waterlaying polyamide fibers from an aqueous dispersion onto the foraminous support of the papermaking machine, said fibers consisting essentially of (A) 10 to 100% by weight of a copolyamide of caprolactam: hexamethylene adipamide in a weight ratio of about 2:13 to 3:1 and (B) to 0% by weight of a polyamide selected from the group consisting of polycaprolactam and polyhexamethylene adipamide; removing water from the fibers to form a non-woven waterlaid web; and heating the waterlaid web in a moist state to a temperature sufficient to soften the copolyamide fibers and cause them to become adherent.

2. A process as claimed in claim 1 wherein the copolyamide consists of about 40 to 75% by weight of caprolactam and about 60 to 25% by weight of hexamethylene adipamide.

G. A process as claimed in claim 1 wherein the copolyamide consists of about 50 to 70% by weight of caprolactam and about 50 to 30% by weight of hexamethylene adipamide.

4. A process as claimed in claim 1 wherein the waterlaid fibers consist essentially of about 25 to 75% by weight of copolyamide (A) and 75 to 25 by weight of polyamide (B).

5. A process as claimed in claim 4 wherein the weight ratio of caprolactam:hexamethylene adipamide in the copolyamide is approximately 7:3.

6. A process as claimed in claim 4 wherein the weight ratio of caprolactarn: hexamethylene adipamide in the copolyamide is approximately 3:2.

7. A process for the production of a fibrous polya-mide laminar structure on a papermaking machine which com prises: waterlaying polyamide fibers from an aqueous dispersion onto the forarninous support of a paperrnaking machine, said fibers consisting essentially of (A) about 25 to 75% by weight of a copolyamide of capro- 1actam:hexamethylene adipamide in a weight ratio of about 2:3 to 3:1 and (B) about 75 to 25% by weight of a polyarnide selected from the group consisting of polycaprolactam and polyhexarnethylene adiparnide; removing water from the fibers to form a non-woven Water laid web; conducting the web in a moist state over a heated rotating cylinder at a temperature sufficient to soften the copolyamide fibers and cause them to become adherent; and drying the heated moist web to remove residual water therefrom.

g 8. A process as claimed in claim 7 wherein the web is conducted over said heated roller in the absence of tension and without substantial contact pressure.

9. The product obtained by the process of claim 1.

References Cited by the Examiner UNITED STATES PATENTS 2,962,762 12/1960 Hartman 162-157 2,999,788 9/ 1961 Morgan 162-146 3,049,466 8/1962 Erlich '1'62157 X 3,068,527 12/1962 Morgan 162-146 X 3,123,518 3/1964 Bundy 162-146 X 3,223,581 12/ 1965 Sommer 162-157 DONALL H. SYLVESTER, Primary Examiner.

H. CAINE, Assistant Examiner. 

1. A PROCESS FOR THE PRODUCTION OF A FIBROUS POLYAMIDE LAMINAR STRUCTURE ON A PAPERMAKING MACHINE WHICH COMPRISES: WATERLAYING POLYAMIDE FIBERS FROM AN AQUEOUS DISPERSION ONTO THE FORAMINOUS SUPPORT OF THE PAPERMAKING MACHINE, SAID FIBERS CONSISTING ESSENTIALLY OF (A) 10 TO 100% BY WEIGHT OF A COPOLYAMIDE OF CAPROLACTAM: HEXAMETHYLENE ADIPAMIDE IN A WEIGHT RATIO OF ABOUT 2:3 TO 3:1 AND (B) 90 TO 0% BY WEIGHT OF A POLYAMIDE SELECTED FROM THE GROUP CONSISTING OF POLYCAPROLACTAM AND POLYHEXAMETHYLENE ADIPAMIDE; REMOVING WATER FROM THE FIBERS TO FORM A NON-WOVEN WATELAID WEB; AND HEATING THE WATERLAID WEB IN A MOIST STATE TO A TEMPERATURE SUFFICIENT TO SOFTEN THE COPOLYAMIDE FIBERS AND CAUSE THEM TO BECOME ADHERENT. 